My husband has taken Allopurinol daily for years and hasn’t had a gout flare-up since he started. He’s never done a prolonged fast, so I can’t speak to that, but he’s always gravitated toward OMAD. The doc who prescribed the Allopurinol is an IF advocate who was the first physician to approve of my husband’s OMAD tendencies, so I’m guessing Allopurinol + fasting is a fine combo.
Uric acid builds up during fasting because the kidneys prioritize expelling ketones so if you’re prone to gout you’re going to get flareups.
The only real long term solution is to limit the purine-rich foods in your diet. Fasting obviously can help with that but if you’re still eating a lot of those foods when not fasting, then it’s just shooting yourself in the foot.